The HDTV (digital high definition wide-screen television) system proposed for use in the United States is totally incompatible with the conventional NTSC broadcast standard in use today. Some primary differences between the two systems are: the HDTV system is digital while the NTSC system is analog, the HDTV system uses a 16:9 aspect ratio while the NTSC system has a 4:3 aspect ratio, the HDTV system will have 1125 television scan lines while the NTSC system has only 525. Thus, if the broadcasting industry immediately adopted the digital HDTV system and abandoned the NTSC system, perfectly serviceable NTSC television receivers purchased within the last few years would be rendered obsolete and useless. To avoid such a result, it is currently envisioned that the transition from conventional analog NTSC-standard broadcasts to digital HDTV television broadcasts will occur over a period which may last some fifteen years to allow for normal attrition of the older NTSC television receivers. During this period, television stations will "simulcast" (i.e., simultaneously broadcast) their programs on two television channels allotted for their use by the FCC (Federal Communications Commission). Each of the two channels will encompass 6 MHz of bandwidth, and need not be contiguous. One of the two channels will be the currently assigned analog NTSC channel, and the other will be assigned for transmission of HDTV material.
It is not expected that all television stations will begin broadcasting HDTV signals on the same day. Instead, the television stations in a given geographic area will most probably begin HDTV transmissions over a period of a few weeks, months, or even years, as their individual finances permit. It is envisioned that television receivers produced during the transition period will include both an HDTV tuner and signal processing section, and an NTSC tuner and signal processing section. In this way, the television receiver could receive and display signals of both systems.
As noted above, each television station in a given geographic area will have two channels assigned to it. It is most likely that these channels will not be contiguous, nor even have a nationwide pattern of channel pairing due to the realities of spectrum crowding. That is, there will be no fixed relationship between the NTSC channel number and the HDTV channel number.
Nevertheless, despite that fact that the signals are carried on two distinct frequencies, and are actually incompatible, it is likely that the viewer will come to regard the transmissions as one television channel because the same television program may be carried simultaneously on both channels. That is, a version of a given television program in the NTSC format may be aired on the NTSC channel at the same time that its corresponding version in the HDTV format is aired on the paired HDTV channel of the same television station. The nontechnical viewer, not fully understanding the system distinctions, will naturally expect to tune his desired television program as easily as he has done in the past.
In order to free the viewer from having to remember that NTSC channel 4 (for example) is paired with HDTV channel 41 (for example), a tuning-setup procedure will be provided so that the viewer may specify which channels are paired in his given geographic area.
PIP (i.e., picture-in-picture, or Pix-in-Pix) circuitry is well-known in modern television receivers. In a pix-in-pix system, the television receiver displays a main image and a small inset image. A common feature of PIP television receivers is "Channel Swap". A channel swap feature allows a viewer to change the positions of the two pictures on the screen. The channel swap feature is performed by baseband signal switching circuitry which switch the former inset image signals to the main image processing circuitry, and switch the former main image to the inset image processing circuitry.
A dual-system (i.e., High Definition TV/NTSC) television receiver which also includes PIP circuitry is known from the Sony KW-3600HD television receiver, produced and marketed in Japan by Sony Corporation in 1990. The KW-3600HD includes circuitry to receive and display NTSC signals and High Definition analog signals (i.e., the MUSE system). The KW-3600HD television receiver is capable of displaying a main picture and a small picture in a variety of modes including a 16:9 HDTV main picture with an NTSC small picture, and an NTSC main picture with an 16:9 HDTV small picture. It is noted that in Japan there are only two MUSE channels, and consequently, they are not "paired" with a corresponding NTSC channel.
As noted above, it is envisioned that television receivers manufactured for use in the United States during the next few years will include both an NTSC tuner and an HDTV tuner. It is reasonable to assume that a dual-system (i.e., HDTV/NTSC) television receiver having PIP capability would restrict the PIP circuitry to process only one of the two systems (i.e., either NTSC signals or HDTV signals) in order to lower the cost to a practical amount. In this regard, it is noted that the KW-3600HD television receiver sold in Japan at a price which was from 20 to 30 times the cost of a conventional console television receiver, and was clearly priced out of the mass market. Unfortunately, if the PIP circuitry were to be designed to process only one of the two systems (i.e., either NTSC signals or HDTV signals), then the act of choosing either of the HDTV system or the NTSC system for the PIP processing circuitry, makes the PIP assembly incompatible with one of the two tuners. In such a case, a problem immediately arises as to how to perform a channel swap feature when the PIP source and the main picture source are on two different and incompatible systems, and when the PIP processing circuitry is limited to operate on only one of the two systems.